Do not expect full state in the "leaves" array

[indrekj]

If there's an update to the state then we always have a "join". If it's just leave, then the state doesn't have to change, and we can use previously known state.

[laurglia]

If we are already changing the Phoenix library, maybe it makes sense to also remove properties besides phx_ref from the leaves here instead of doing it in the JSON encoder?

[indrekj]

Hm. That would mean changing the server part as well. The current onChange part is our invention anyway, so I was more comfortable changing it. At least it doesn't move our fork more away from the upstream.